Evidence from previous autoradiographic studies in our laboratory have demonstrated atrial myocardiocytes are target cells for estradiol. The purpose of this project is to follow up, that is, search for and characterize estrogen and other sex and adrenal steroid hormone target sites in heart tissues of representatives of different mammalian species, using dry-mount and thaw-mount autoradiography. The cellular and subcellular localization of tritium labeled cardiac glycosides will also be studied in a similar fashion, and the results compared with those obtained with 3H estradiol and other steroid hormones. Competition studies between hormonal steroids and cardiac glycosidal steroids are aimed at elucidating uptake characteristics and specificity. Possible effects of estradiol on the production of atrial granules will be assessed with the use of labeled protein precursors, employing light and electron microscopic autoradiography.